Caption: Cross-section through the trunk of an elm tree, Ulmus sp., showing the heartwood (darker, centre), sapwood (paler, outer ring) and bark. The sapwood is saturated with water and forms the tree's conducting tissue, whereas the heartwood is a completely lifeless, dry tissue which functions only as a structural support. The tree's annual growth rings are also clearly visible. A count of the rings would indicate the age of the tree.